Hydroxysteroid dehydrogenases (HSD) were evaluated histochemically in tissues representing seven regions of the epididymides from twenty-five neopubertal and three adult bulls. The epithelium of the epididymides was devoid of HSD activity against each of twenty steroid substrates tested; only 3β,16β-dihydroxy-5-androsten 3-methyl ester was dehydrogenated. In luminal spermatozoa, however, 3α-, 3β-, 16β- and 17β-HSD activities were detected in sections from most neopubertal and all adult bull epididymides incubated with 5β-androsterone, 5β-epiandrosterone, 3β,16β-dihydroxy-5-androsten 3-methyl ester and 5β-dihydrotestosterone. All other substrate incubations were negative. There was no age-associated difference in demonstrable HSD activity of epididymal spermatozoa. In smears of unwashed testicular spermatozoa from adult bulls, 3α-HSD activity was most conspicuous on the sperm head and cytoplasmic droplet, 3β-HSD activity was most prevalent in the mid-piece and the 17β-HSD activity predominated in the mid-piece and the cytoplasmic droplet.